1. Field of the Invention
The present invention relates generally to a liquid crystal display device and a driving method thereof, and more particularly to a liquid crystal display device which is driven in an active matrix method, and a driving method thereof.
2. Description of the Related Art
In recent years, mobile products in which liquid crystal panels are built, such as small-sized game machines, portable PCs and mobile phones, have been quickly gaining in popularity.
In general, the liquid crystal display panel is configured such that a liquid crystal layer is held between an array substrate and a counter-substrate. In the case where the liquid crystal display panel is of an active matrix type, the array substrate includes a plurality of pixel electrodes which are arranged substantially in a matrix, a plurality of gate lines which are disposed along rows of the plural pixel electrodes, a plurality of source lines which are disposed along columns of the plural pixel electrodes, and a plurality of pixel switching elements which are disposed near intersections of the plural gate lines and plural source lines.
The respective gate lines are connected to a gate driver which drives the gate lines. The respective source lines are connected to a source driver which drives the source lines. The gate driver and source driver are controlled by a control circuit. Each of the pixel switching elements is composed of, e.g. a thin-film transistor (TFT). When the associated gate line is driven by the gate driver, the pixel switching element is rendered conductive, thereby applying a pixel voltage, which is set on the associated source line by the source driver, to the associated pixel electrode.
The counter-substrate is provided with a common electrode which is opposed to the plural pixel electrodes disposed on the array substrate. A liquid crystal pixel is constituted by a pair of each pixel electrode and the common electrode, together with a pixel region which is a part of the liquid crystal layer that is interposed between these paired electrodes. A driving voltage for the pixel is a difference between a pixel voltage, which is applied to the pixel electrode, and a common voltage which is applied to the common electrode. Even after the pixel switching element is turned off, the driving voltage is held between the pixel electrode and the common electrode.
Alignment of liquid crystal molecules in the pixel region is set by an electric field which corresponds to the driving voltage. Thereby, the transmittance of the pixel is controlled. The polarity reversal of the driving voltage is executed, for example, by cyclically reversing the polarity of the pixel voltage in relation to the common voltage. Thus, the direction of electric field is reversed to prevent non-uniform distribution of liquid crystal molecules in the liquid crystal layer.
In the field of large-sized liquid crystal TVs, liquid crystal display panels of an OCB (Optically Compensated Bend) mode, which has a high liquid crystal responsivity that is needed for motion video display, have begun to be adopted. This liquid crystal display panel performs a display operation by transitioning the alignment state of liquid crystal molecules from splay alignment to bend alignment in advance. In this case, if a voltage-off state or a nearly voltage-off state continues for a long time, the bend alignment reversely transitions to the splay alignment. In this type of liquid crystal display panel, black-insertion driving is used in order to prevent the reverse transition to the splay alignment.
On the other hand, in the prior art, there is proposed a liquid crystal display device which is configured such that a plurality of write periods are provided in one horizontal period and video signals are output to different source lines in these write periods in a distributed manner (see Jpn. Pat. Appln. KOKAI Publication No. 2004-219823).
When selective driving is performed, only a selected selection switch of a source line output is turned on, and a desired signal voltage is written on the associated source line. At this time, if black-insertion driving is to be further executed, a plurality of selection switches are simultaneously turned on to start write of a black-level signal voltage. Thereafter, the plural selection switches are simultaneously turned off to stop the write of the black-level signal voltage. In this case, there may arise such a problem that the load on the source driver varies before and after the turn-off of the selection switches and a voltage signal, which exceeds a rated voltage, may occur at a timing when the channels of the plural selection switches are simultaneously turned on/off.